II-VI based light emitting semiconductor device

ABSTRACT

The invention provides a light emitting semi conductor device comprising a zinc magnesium oxide based layer as active layer, wherein the zinc magnesium oxide based layer comprises an aluminum doped zinc magnesium oxide layer having the nominal composition Zn-xMgxO with 1-350 ppm Al, wherein x is in the range of 0&lt;x≦0.3. The invention further provides a method for the production of such aluminum doped zinc magnesium oxide, the method comprising heat treating a composition comprising Zn, Mg and Al with a predetermined composition at elevated temperatures, and subsequently annealing the heat treated composition to provide said aluminum doped zinc magnesium oxide.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§371 of International Application No. PCT/IB2013/055325, filed on Jun.28, 2013, which claims the benefit of U.S. Patent Application No.61/665,968, filed on Jun. 29, 2012 and U.S. Patent Application No.61/739,165, filed on Dec. 19, 2012. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to a II-VI based light emitting semiconductordevice, a luminescent material, a method for the production of suchluminescent material, as well as to a method for the production of aII-VI semiconductor layer for such semiconductor device.

BACKGROUND OF THE INVENTION

Wide-band gap II-VI compounds are expected to be one of the most vitalmaterials for high-performance optoelectronics devices such aslight-emitting diodes (LEDs) and laser diodes (LDs) operating in theblue or ultraviolet spectral range. Thin films were commonly grown usingthe conventional vapor-phase epitaxy (VPE) method. With the developmentof science and technology, new and higher requirements arose formaterial preparation. For this reason, novel epitaxial growth techniqueswere developed, including hot-wall epitaxy (HWE), metal organic chemicalvapor deposition (MOCVD), molecular-beam epitaxy (MBE), metal organicmolecular-beam epitaxy (MOMBE) and atomic layer epitaxy (ALE). Usingthese growth methods, film thickness can be controlled and quality canbe improved. Examples of II-VI semiconductors are ZnS, ZnO, ZnSe, ZnTe,and CdTe.

Zinc oxide semiconductor materials comprising zinc and oxygen asconstituent elements have attracted considerable attention since theycan emit not only blue light but also near ultraviolet rays of 400nanometers or less because of their wide band gap similarly tosemiconductor materials such as gallium nitride and the like. Further,their applications to photo detector, piezoelectric device, transparentconductive electrode, active device and the like have also been expectedwithout being limited to light emitting device. To form such a zincoxide semiconductor material, various methods such as MBE method usingultra-high vacuum, sputtering, vacuum evaporation, sol-gel process,MO-CVD method, and the like have been conventionally examined. Withrespect to the light emitting device, the MBE method using ultra-highvacuum is widely used from the viewpoint of crystallinity.

Further, U.S. Pat. No. 4,278,913 describes a zinc oxide-based phosphoremits yellow light of high luminance under excitation of low-velocityelectrons: xM^(II)F₂.yM^(III)F₃.ZnO wherein M^(II) is at least onedivalent metal selected from the group consisting of beryllium,magnesium, calcium, strontium, barium, zinc and tin, M^(III) is at leastone trivalent metal selected from the group consisting of aluminum,gallium, indium, thallium, yttrium and antimony, and x and y are numberssatisfying the conditions of 0.0001≦x+y≦0.1, 0≦x and 0<y. The zincoxide-based phosphor is used as a fluorescent screen of a low-velocityelectron excited fluorescent display device.

SUMMARY OF THE INVENTION

Currently, the lighting world is in the middle of a transition from theincandescent bulbs and (compact) fluorescent lamps towards solid-statelighting, mostly provided by light emitting diodes (LEDs). The majorityof the LEDs in market are based on gallium nitride (GaN). While GaN isan excellent emitter, it does suffer from several drawbacks. The mainissue is the susceptibility to defects in the crystal lattice that aregenerally detrimental for the emissive properties of GaN layers. Yet,GaN is the most suitable III-V material for LED fabrication because isactually one of the more defect-tolerant III-V materials. In order toprevent emission loss, the defect concentration has to be kept low bygrowing the GaN layers epitaxially. Epitaxial growth however preventsfabrication of large area devices. Additionally, the GaN covered wafersare generally cut up into small parts (typically 1×1 mm) to ensure anacceptable yield in functional devices, and to ensure optimum use ofmaterials, because of the fact that gallium is a relatively scarce andexpensive element.

The requirement for small areas has several disadvantages. In order tohave enough light output GaN LEDs are generally operated at high powerdensities leading to heating of the devices which decreases theirefficiency and requires the use of heat dissipation mechanisms such asheat sinks. The high light output from a small area effectively makesthem point sources, which makes it uncomfortable to look directly intowhen used for general lighting applications. In fact, high power LEDsare classified on par with lasers with respect to eye safety. Therefore,for lighting applications, some kind of light spreading and glarereduction is generally required. An approach to solve these issues is tohave large light emitting surfaces that can be driven at much lowerpower densities. As mentioned above, a large-area GaN light source iscurrently impossible and does not exist.

One of the reasons for the vulnerability for defects in the GaN crystallattice stems from the low exciton binding energy, which is below kT.This low value means that at room temperature, excitons are likely tosplit up in separate electrons and holes before they have a chance toradiatively recombine. The separated charge carriers are then trapped atdefect sites, leading to non-radiative decay. Obviously, this processintensifies at the elevated temperatures that GaN LEDs are commonlyoperated at.

On the other side of the spectrum are organic LEDs (OLEDs) with anexciton binding energy of 0.5 eV, far larger than kT, enabling lightemission from an essentially amorphous medium which makes large arealighting applications possible. OLEDs however require (expensive)encapsulation due to the reactive nature of the electrode materialsused.

Zinc oxide has long been studied as a material that may have the best ofboth worlds. Like GaN, it is a wide band gap semiconductor (˜3.3 eV),but ZnO has a high exciton binding energy of 60 meV (2.4 times kT atroom temperature). This value means that defects should be lessdetrimental to light emission, thereby enabling a switch from epitaxialgrowth methods towards cheaper, large area deposition techniques likesputtering that generally result in polycrystalline layers. Furthermore,ZnO is cheap, abundant and highly stable making it an attractive choiceas a potential light emitting material in large area devices.

Zinc oxide can be applied using large area deposition techniques likesputtering, which generally results in polycrystalline layers.Furthermore, ZnO is cheap, abundant and highly stable, making it anattractive choice as a potential light emitting material in large areadevices. However, despite these promises, ZnO has a few issues as apotential phosphor that have not been solved yet. Firstly, the main(near band gap) emission is in the UV (˜385 nm) and secondly the quantumefficiency of this emission is very low. Up to 3% efficiency has beenreported from powder at room temperature, but generally lower values areobserved.

A well known additive is sulfur, which results in a strong, broad bandemission from ZnO centered around 500 nm with a quantum efficiency of˜50%. Although the preparation of highly luminescent ZnO:S powder israther straightforward, the deposition of thin films of a similarcomposition is troublesome.

Therefore, there is an interest in additives for ZnO that improve thevisible emission and quantum efficiency of the material, while beingcompatible with large area deposition techniques like sputtering. Hence,it is an aspect of the invention to provide an alternative (lightemitting) semiconductor device, which preferably further at least partlyobviates one or more of above-described drawbacks. It is further anaspect of the invention to provide an alternative luminescent material,which preferably further at least partly obviates one or more ofabove-described drawbacks. Further, it is an aspect to provide a methodfor the production of such luminescent material, especially in the formof a layer on a substrate that can be used as active layer in suchalternative semiconductor device.

In a first aspect, the invention provides a light emitting semiconductor device comprising a stack, the stack comprising a cathode(which may especially be a cathode layer), a semiconductor layercomprising an emissive (oxidic) material having an emission in the rangeof 300-900 nm, an (oxidic) insulating layer, and an anode (which mayespecially be an anode layer), wherein the cathode is in electricalcontact with the semiconductor layer, wherein the anode is in electricalcontact with the insulating layer, such as a metal oxide layer, andwherein the insulating layer has a thickness in the range of up to 50 nm(i.e. >0 nm and ≦50 nm).

This approach is a realization of the diode by incorporation of aninsulating layer in the device stack, i.e.metal-insulator-semiconductor-metal (MISM) diode. The cathode or anodecan in principle be of any material that is suitable as cathode or anodematerial, respectively. Especially, at least one of cathode or anode istransmissive. In an embodiment, the cathode comprises ZnO doped withaluminium or indium tin oxide (ITO). Hence, in an embodiment, thecathode is transmissive. Herein, the term “transmissive” indicates thatthe layer is transmissive for emission of the active layer. In a furtherembodiment, the anode may be a noble metal, such as gold or platinum, ora combination thereof.

Suitable materials for the semiconductor layer may especially be anemissive material selected from the group consisting of oxides, sulfidesor selenides of zinc or cadmium, such as zinc oxide, zinc magnesiumoxide, zinc sulfide, zinc selenide, cadmium oxide, cadmium sulfide, andcadmium selenide, especially ZnO, (Zn,Mg)O, ZnS, ZnSe, CdO, CdS, CdSe.Further, also oxysulfides may be applied, like gadolinium oxy sulfide.Further, also doped version of these materials may be applied, likeZnO:Al, (Zn,Mg)O:Al, ZnO:Mn, (Zn,Mg)O:Mn, etc. Hence, in an embodiment,the emissive material is selected from the group consisting of ZnO,(Zn,Mg)O, ZnS, ZnSe, CdO, CdS, CdSe, and doped variants of any of these,(like ZnO:Al, (Zn,Mg)O:Al, ZnO:Mn, (Zn,Mg)O:Mn, etc.). Also othersemiconducting materials may be applied that show emission in thevisible. Optionally, also tellurides, like ZnTe, might be applied.

Hence, in a specific embodiment, the semiconductor layer comprises anemissive material selected from the group consisting of zinc (magnesium)oxide and cadmium oxide. The semiconductor layer is herein alsoindicated as “active layer”. In an embodiment, the layer is anon-granular layer, such as a layer obtainable by CVD or sputtering (andannealing), or other techniques known in the art, such as describedherein. However, in another embodiment, the layer comprises aparticulate layer, such as a layer comprising semiconductingnanoparticles. In another embodiment, the layer comprises (semiconducting) quantum dots. The layer is especially a continuous layer,with a porosity of at maximum 5%.

The term “(oxidic) emissive material” indicates that the emissivematerial may be a metal oxide material, such as ZnO. However, theemissive layer may also comprise a sulphide or selenide emissivematerial, etc., see also above.

The semiconductor layer comprises an emissive material having anemission in the range of 300-900 nm, such as in the range of 300-800 nm,like 400-700 nm. Especially, the semiconductor layer has at least partof its emission in the visible part of the optical spectrum. Likewisethis may apply to the luminescent material as described below.

The term white light herein, is known to the person skilled in the art.It especially relates to light having a correlated color temperature(CCT) between about 2000 and 20.000 K, especially 2700-20.000 K, forgeneral lighting especially in the range of about 2700 K and 6500 K, andfor backlighting purposes especially in the range of about 7000 K and20.000 K, and especially within about 15 SDCM (standard deviation ofcolor matching) from the BBL (black body locus), especially within about10 SDCM from the BBL, even more especially within about 5 SDCM from theBBL. The terms “violet light” or “violet emission” especially relates tolight having a wavelength in the range of about 380-440 nm. The terms“blue light” or “blue emission” especially relates to light having awavelength in the range of about 440-490 nm (including some violet andcyan hues). The terms “green light” or “green emission” especiallyrelate to light having a wavelength in the range of about 490-560 nm.The terms “yellow light” or “yellow emission” especially relate to lighthaving a wavelength in the range of about 560-590 nm. The terms “orangelight” or “orange emission” especially relate to light having awavelength in the range of about 590-620. The terms “red light” or “redemission” especially relate to light having a wavelength in the range ofabout 620-750 nm. The terms “visible” light or “visible emission” referto light having a wavelength in the range of about 380-750 nm.

Specific embodiments of the active layer (material) are elucidatedbelow, but first the insulating layer is discussed.

As indicated above, especially good results are obtained due to thepresence of the (oxidic) insulating layer, which may also be indicatedas barrier layer. The term “oxidic insulating layer” indicates that thebarrier layer is a metal oxide layer. This layer may also comprise aplurality of layers, optionally of different metal oxides. The term“metal oxide” may also refer to a mixed metal oxide. This insulatinglayer should preferably not influence the optical properties of theactive layer. In other words, the insulating layer should preferably notinfluence the emission position of the emission band of the activelayer. Especially, the insulating layer or barrier layer does notsubstantially react with the active layer, also not during applicationof the insulating layer on the active layer or during application of theinsulating layer on the active layer (“inverted structure”). Hence, itis highly desirable to have a blocking layer that is stable in air anddoes not intermix with the underlying active layer, such as ZnO phosphorlayer (see below) upon annealing (see also below). A good candidate forsuch layer is ZrO, which is a stoichiometric oxide with very limitedsolubility in ZnO. Especially, the oxidic insulating layer is selectedfrom the group consisting of SiO₂, MgO, SrTiO₃, ZrO₂, HfO₂, and Y₂O₃. Ina further variant, the insulating layer is a high bandgap dielectricalmaterial, such as with a bandgap of at least 5 eV, especially at least5.5 eV. The insulating layer may also comprise a non-oxidic material.

It is further desired that the position of the valence band andconduction band of the insulating layer is positioned such thatconduction band of the (material of the) insulating layer is higher thanof the conduction band of the (material of the) active layer. Further,the position of the valence band of the (material of the) insulatinglayer may be in the vicinity of the valence band of the (material ofthe) active layer.

Especially, the emissive material has a conduction band at CBp eV and avalence band at VBp eV from the vacuum level, with CBp>VBp, wherein thebarrier layer has a conduction band at CBb eV and a valence band at VBbeV from the vacuum level, with CBb>VBb, wherein CBb>CBp, especiallywherein CBb≧CBp+0.25 eV. Further, in an embodiment especiallyVBb≦VBp+1.5 eV, even more especially VBb≦VBp+1 eV. The vacuum level at 0V is taken as reference.

Vc and Vb usually have negative values. Therefore, when Vc>Vb thisimplies that that |Vc| is smaller than |Vb|. Such conditions may givebest results in terms of efficiency of the device. For instance, aconduction band of the barrier layer that is too close to, or even belowthe conduction band of the active layer may lead to an inefficient lightemission in comparison with a barrier layers as indicated above, becausethe barrier required for blocking electron transport in the active layerhas been disappeared. Especially, CBb≧CBp+0.35 eV, even more especially,CBb≧CBp+0.5 eV. Further, as indicated above especially VBb≦VBp+1.5 eV,even more especially VBb≦VBp+1 eV.

To give an example, the emissive material (of the active layer) may havea conduction band at −4 eV and a valence band at −7 eV; hence CBp>VBp.Further, the barrier layer may e.g. have a conduction band at −3 eV anda valence band at e.g. −6 eV, or −8 eV. Hence, CBb>VBb. Further, alsoCBb≧CBp+0.25 eV and VBb≦VBp+1 eV apply.

Especially, the thickness of the insulating layer is within thetunneling limit. Hence, the insulating layer has a thickness which isespecially equal to or smaller than 50 nm, such as equal to or smaller30 nm, like especially in the range of 2-30 nm, like at least 4 nm.

Here, we present also a novel class of zinc oxide based phosphors withenhanced quantum efficiency and emission in the visible part of thespectrum, that are also amenable to robust, large area thin layerdeposition techniques such as sputtering, and which may also be used asmaterial for an active layer in above described device. The enhancedemission is achieved by incorporating both magnesium and a trace amountof aluminum, followed by annealing in a non-reducing atmosphere,especially in air. The enhanced emission does not seem to stem fromeither the Al or Mg themselves, but is attributed to radiating defectsin the (modified) ZnO lattice, the nature and number of which arethought to be influenced by the additives. The presence of both Al andMg seem to have a synergistic effect. These ZnO based materials areprospective candidates for the emissive layer in large area LEDs. Theemissive layer is herein also indicated as “active layer”. The term“active layer” indicates that this layer in the semiconductor devicewill show the desired luminescence (emission), when the semiconductordevice is driven under the right conditions. The layer is especially athin film, such as having a thickness in the range of 50 nm-1000 nm (1μm). The layer is especially a continuous layer, with a porosity of atmaximum 5%.

In a further aspect, the invention provides a (light emitting)semiconductor device (herein also indicated as “device”) comprising azinc oxide or zinc magnesium oxide based layer, especially a zincmagnesium oxide based layer, as active layer, wherein the zinc magnesiumoxide based layer comprises (or especially consists of) an aluminumdoped zinc magnesium oxide layer having 1-350 ppm Al. The zinc magnesiumoxide of the aluminum doped zinc magnesium oxide layer is of the typeZnO; thus more precisely (Zn,Mg)O; i.e. especially a (Zn,Mg)O:Al layeris provided. Instead of, or in addition to the Al dopant, also otherdopants may be applied, like Mn (manganese).

Especially, the invention provides a semiconductor device comprising azinc magnesium oxide based layer as active layer, wherein the zincmagnesium oxide based layer comprises (even more especially consists of)an aluminum doped zinc magnesium oxide layer having the nominalcomposition Zn_(1-x)Mg_(x)O with 1-350 ppm Al, wherein x is in the rangeof 0<x≦0.3. The phrase “Zn_(1-x)Mg_(x)O with 1-350 ppm Al” may also be,as known in the art, indicated as Zn_(1-x)Mg_(x)O:Al (1-350 ppm). Here,the term “nominal composition” is applied, as the composition hereinindicated relates to the composition as weighed in. Hence, the nominalcomposition might also be indicated as “(1−x)ZnO*xMgO with 1-350 ppmAl”.

It appears that a relative highly efficient active layer is provided,that has the desired properties in respect of efficiency and electricalresistance. Further, such layer may be produced relatively easy. Layerswithout Mg or without Al are less efficient. Further, layers having ahigher Al content may have undesired conductive properties.

It seems that Mg (magnesium) may at least partly be built in the ZnOlattice (alternatively, one may say that MgO dissolves in the ZnOlattice). The amount of Mg in the nominal composition is indicated withx, which is especially in the range of 0<x≦0.3, and even more at maximum0.2. In the range of 0.02<x≦0.2 best optical properties may be obtained.The intrinsic value for x may especially be 0.1-0.2, like about 0.15 fora layer, whereas for a poly crystalline material, the value for x mayespecially be 0.04 or lower. The intrinsic value refers to the x-valueof the mixed oxide. The presence of Mg in the zinc oxide can bedetermined from XRD (x-ray diffraction), or SIMS, RBS or ICP/MS, seealso below.

With respect to Al (aluminum), it seems that 1-350 ppm (parts permillion) Al, especially 1-200 ppm, even more especially 1-100 ppm, givegood optical properties and also does not lead to a high conductivity,which is not desired, and which may occur when high Al amounts are used.An amount of Al in the range of 2-100 ppm, such as 5-100 may beespecially suitable, even more a range of 2-80 ppm, such as 2-70, suchas 10-60 ppm, like 20-60 ppm, especially like 30-50 ppm can be used. Inan embodiment, the aluminum content is at least 10 ppm. Aluminum maypartly be present in the zinc magnesium oxide lattice as dopant. Al mayreplace Zn or Mg lattice positions or may form or occupy interstitialpositions in the lattice. The presence of Al can be reflected in SIMS(Secondary Ion Mass Spectrometry) or RBS (Rutherford backscattering) ofthe material. Optionally also laser ablation with ICP/MS (InductivelyCoupled Plasma Mass Spectrometry) can be used to detect the presence ofAl. The ppm value of the dopant relates to the total molar amount of thesystem. Hence, 10 μmol Al in 1 mole Zn_(1-x)Mg_(x)O:Al will lead to avalue of 10 ppm Al, i.e. Zn_(1-x)Mg_(x)O:Al (10 ppm).

Hence, in a specific embodiment the zinc magnesium oxide contains 5-100ppm Al, wherein x is in the range of 0.02<x≦0.2 (nominal composition).Further, especially the sulfur content in the zinc magnesium oxide(based layer) is lower than 50 ppm. Higher sulfur contents may lead tosystems that cannot easily form the desired composition of the layer.For semiconductor applications, the layer thickness of the aluminumdoped zinc magnesium oxide layer may be in the range of 50-1000 nm, suchas at least 100 nm. The way in which such active layers may be formed isfurther elucidated below.

A semiconductor device, with such aluminum doped zinc magnesium oxidelayer active layer can advantageously be used to generate visible light,especially having a dominant wavelength in the wavelength range of500-650 nm. The term dominant wavelength indicates that the emissionintensity maximum is found within the indicated spectral region.Further, it appears that the aluminum doped zinc magnesium oxide layerhaving the nominal composition Zn_(1-x)Mg_(x)O with 1-350 ppm Al,wherein x is in the range of 0<x≦0.3, can advantageously be used asactive layer in a large area LED, the large area LED at least having adie area of at least 1 cm².

The premise of ZnO is application in large area lighting due to theexciton binding energy of 60 meV being larger than kT. In III-V LEDs(such as GaN), the binding energy is smaller than kT. Hence, for a highphotoluminescence efficiency non-radiative defects have to be avoided.Epitaxially grown thin films are required; the technology cannot bescaled up to large areas. In OLEDs however, the exciton binding energyis about 0.5 eV. Light can be generated in amorphous films that arefabricated by roll-to-roll processing. The challenge for OLEDs is costprice and encapsulation.

The large binding energy makes ZnO a defect tolerant host material.Epitaxial thin films are not needed; a high efficiency might be obtainedwith polycrystalline thin films deposited over large area. Numerouspapers report light emission from polycrystalline oxide LEDs fabricatedby various deposition methods. The present efficiency is low, but thereis not necessarily a fundamental limitation. When the efficiency can beoptimized it will pave the way for large area solid state lighting. Theadvantages are low-cost and environmentally stable diodes that can befabricated over a large area with industrially established depositiontechniques.

As may be known in the art, the ZnO-based layer may be sandwichedbetween electrodes of the semiconductor device. Further modification ofthe ZnO-based layer to provide the semiconductor device may also beincluded. For instance, optionally one or more electron or hole blockinglayers may be applied. This may improve efficiency. One or more electronor hole blocking layers may be arranged at different positions withinthe stack.

Hence, in an embodiment, the invention provides a light emittingsemiconductor device, wherein the semiconductor layer comprises aluminumdoped zinc magnesium oxide layer having 1-350 ppm Al. Especially, thesemiconductor layer has a nominal composition Zn_(1-x)Mg_(x)O with 1-350ppm Al, wherein x is in the range of 0<x≦0.3.

The stack especially comprises a cathode, a semiconductor layercomprising an emissive (oxidic) material, an (oxidic) insulating layer,and an anode (and a support). Optionally, between the support and thecathode or anode, one or more other (functional) layers may be present.Further, optionally between the cathode and the semiconductor layer oneor more other functional layers may be present. Especially, between thesemiconductor layer and the (oxidic) insulating layer (the barrierlayer), and between the barrier layer and the anode, no further otherlayers are present. However, in addition to (or alternative to) theinsulating layer between the semiconductor layer and the anode, one ormore further (or other) other blocking layers may be present in thedevice stack that are not necessarily in contact with the anode (orcathode). However, the one at the anode is especially applied, to helpand facilitate hole injection.

The invention also provides the (particulate) luminescent material perse, and thus not only as active (thin) layer in a semiconductor device.Hence, as indicated above, the invention also provides a luminescentmaterial comprising zinc magnesium oxide doped with Al. The zincmagnesium oxide is of the type ZnO; hence, especially (Zn,Mg)O:Al isprovided. Hence, the invention also provides a luminescent materialcomprising zinc magnesium oxide doped with Al having the nominalcomposition Zn_(1-x)Mg_(x)O with 1-350 ppm Al, wherein x is in the rangeof 0<x≦0.3. This may be a particulate or granular material. Preferredranges with respect to Mg content and Al content are the same asindicated above for the aluminum doped zinc magnesium oxide layer. Forinstance, the zinc magnesium oxide (luminescent material) may contain5-40 ppm Al, wherein x is in the range of 0.02<x≦0.2.

The fact that the nominal composition “Zn_(1-x)Mg_(x)O” is applied doesnot exclude (small) non-stoichiometric variations, such as in the orderof at maximum 5%. Further, this chemical nominal composition does notexclude the presence of other dopants than aluminum (and magnesium). Forinstance, also sulfur might be present. In an embodiment however, nosulfur is present.

The invention also provides a method for the production of the lightemitting semiconductor device as described herein. Hence, in a furtheraspect the invention provides a method for producing a light emittingsemi conductor device, the method comprising providing a support andforming a stack on the support, wherein the stack comprises a cathode, asemiconductor layer comprising an emissive material having an emissionin the range of 300-900 nm, an (oxidic) insulating layer, and an anode,wherein the cathode is in electrical contact with the semiconductorlayer, wherein the anode is in electrical contact with the insulatinglayer, and wherein the insulating layer has a thickness in the range ofup to 50 nm. Such device may be made with conventional semiconductorproduction technologies, though formation of the semiconductor layer,i.e. the active layer, may especially be done via pulsed laserdeposition (PLD) and radio frequency (RF) sputtering. Hence, in anembodiment the formation of the layer comprises a deposition techniqueselected from the group consisting of pulsed laser deposition (PLD) andradio frequency (RF) sputtering. Other techniques that may be used aswell are e.g. atomic layer deposition (ALD) chemical vapor deposition(CVD) and its variants of CVD method such as metal-organic CVD (MOCVD)or plasma enhanced CVD (PECVD), hydrothermal growth, spray pyrolysis,etc.; in general any physical and chemical evaporation technique may beapplied. Likewise, this may apply for one or more of the other layers inthe stack.

In an embodiment, the production comprises forming the cathode on thesupport, the semiconductor layer on the cathode, the (oxidic) insulatinglayer on the semiconductor layer, and the anode on the (oxidic)insulating layer, followed by annealing the stack, wherein annealing isperformed at a temperature in the range of 400-1100° C. However,inverted building is also possible.

As indicated above, for the conduction band and valence band of theinsulating layer especially applies CBb>CBp, especially CBb≧CBp+0.25and/or VBb≦VBp+1.5 eV, even more especially VBb≦VBp+1 eV. CBb refers tothe conduction band of the barrier; CBp refers to the conduction band ofthe active layer (phosphorescent layer); likewise, VBb refers to thevalence band of the barrier and VBp refers to the valence band of theactive layer. For instance, the (oxidic) insulating layer is selectedfrom the group consisting of SiO₂, MgO, SrTiO₃, ZrO₂, HfO₂, and Y₂O₃.Suitable emissive materials are also defined above.

In a specific embodiment, the semiconductor layer (thus formed) has thenominal composition Zn_(1-x)Mg_(x)O with 1-350 ppm Al, wherein x is inthe range of 0<x≦0.3. In a further embodiment, the method comprises (a)providing a composition comprising Zn, Mg and Al having the nominalcomposition Zn_(1-x)Mg_(x)O with 1-350 ppm Al, wherein x is in the rangeof 0<x≦0.3, optionally heat treating this composition at elevatedtemperatures, and (b) subsequently annealing the optionally heat treatedcomposition to provide said aluminum doped zinc magnesium oxide.

As indicated above, the invention also provides a method for theproduction of an aluminum doped zinc magnesium oxide, such as describedabove. Hence, in a further aspect, the invention provides a method forthe production of an aluminum doped zinc magnesium oxide, the methodcomprising (a) providing a composition comprising Zn, Mg and Al withhaving the nominal composition Zn_(1-x)Mg_(x)O with 1-350 ppm Al,wherein x is in the range of 0<x≦0.3, optionally heat treating thiscomposition at elevated temperatures, and (b) subsequently annealing theoptionally heat treated composition to provide said aluminum doped zincmagnesium oxide.

Even more especially, the invention provides a method for the productionof an aluminum doped zinc magnesium oxide having the nominal compositionZn_(1-x)Mg_(x)O with 1-350 ppm Al, wherein x is in the range of 0<x≦0.3.This may include a method to generate a (particulate) luminescentmaterial, but this may also include a method to produce a thin layer ona substrate. Especially, the method comprises heat treating (especiallyunder oxidative conditions) a composition comprising Zn, Mg and Al witha predetermined nominal composition at elevated temperatures, andsubsequently annealing the heat treated composition to provide saidaluminum doped zinc magnesium oxide. The phrase “a compositioncomprising Zn, Mg and Al” may especially refer to one or more compoundscomprising Zn, Mg and/or Al, respectively. These may also be indicatedas precursor(s), see below.

The term composition may in an embodiment relate to a combination of oneor more precursors of the luminescent material, such as metal oxides, ormetal salts, like nitrates, sulfates, chlorides, fluorides, bromides,hydroxides, carboxylates such as oxalates, etc. etc. Optionally also asulfide (or even optionally a selenide and/or a telluride), such as zincsulfide might be applied as precursor. Especially, one or more of ametal oxide, a nitrate, a chloride, a hydroxide, and a carboxylate (suchas an oxalate) are applied. Combinations of two or more of suchprecursor types may also be applied. Due to the heat treatment, thealuminum doped zinc magnesium oxide may be formed, but especially thematerial may be formed during annealing. The heat treatment andannealing may in an embodiment be performed until at least a polycrystalline material is formed.

In another embodiment, the composition may be formed on a substrate.This may be done at elevated temperatures. For instance, the substratemay be heated. Hence, in a specific embodiment of the method, the methodcomprises forming an aluminum doped zinc magnesium oxide based layerhaving the nominal composition Zn_(1-x)Mg_(x)O with 1-350 ppm Al,wherein x is in the range of 0<x≦0.3, the method comprising forming alayer comprising the composition comprising Zn, Mg and Al with thepredetermined nominal composition on a substrate at elevatedtemperatures, and annealing the thus formed layer to provide thealuminum doped zinc magnesium oxide based layer. For such an embodiment,the formation of the layer comprises a deposition technique selectedfrom the group consisting of pulsed laser deposition (PLD) and radiofrequency (RF) sputtering. However, also other deposition techniques maybe applied (see also above).

As target material, the oxides may be applied, or mixed oxides may beapplied. Especially, as target material (crystalline) aluminum dopedzinc magnesium oxide is applied. Hence, the method may especiallycomprise depositing of the layer of said zinc magnesium oxide on thesubstrate by pulsed laser deposition or RF sputtering of an aluminumdoped zinc magnesium oxide having the nominal compositionZn_(1-x)Mg_(x)O with 1-350 ppm Al (i.e. here target material), wherein xis in the range of 0<x≦0.3 (nominal composition). With pulsed laserdeposition (PLD) and radio frequency (RF) sputtering, a layer may bedeposited on the substrate, having the desired composition. In this way,a II-VI semiconductor layer for a semiconductor device may be produced.

The term “the predetermined nominal composition” especially relates tothe fact that starting components or a composition are composed in sucha way, that the ratio of the elements may lead to the desiredcomposition of the end product, i.e. aluminum doped zinc magnesium oxidehaving the nominal composition Zn_(1-x)Mg_(x)O with 1-350 ppm Al,wherein x is in the range of 0<x≦0.3. As indicated above, the valuesthat can be derived from the formula “Zn_(1-x)Mg_(x)O with 1-350 ppm Al”refers to the nominal composition that is weighed out, and which formsthe zinc magnesium oxide. The formed material may in addition to thezinc magnesium oxide, also optionally comprise (remaining) MgO.

With respect to deposition (of the semiconductor layer; i.e. the activelayer), the deposition is especially performed during a deposition time,wherein during at least part of the deposition time the substrate ismaintained at a temperature of at least 450° C. for RF sputtering or atleast 500° C. for pulsed laser deposition. With the indicatedtechniques, layers may be grown at a rate of about 0.3-1 nm/s, such as04-0.8 nm/s, like about 0.6 nm/s.

The (first) heat treatment is especially at a temperature of at least450° C., although for the synthesis of the luminescent material, even atemperature of at least 900° C., such as at least 1100° C. may bechosen. For instance, in the case of the heat treatment to provide theluminescent material, the temperature may be in the range of 1000-1800°C. Thereafter, the material may be cooled down, ground (in case of aparticulate material), and be subject to the annealing. In case ofmaking the semiconductor layer, the (first) heat treatment will ingeneral be at least 450° C., but not higher than 800° C. However, in yetanother embodiment, deposition is done at a substrate at a lowertemperature than 450° C. Optionally, the substrate may even be at roomtemperature. Especially, however, the substrate is at elevatedtemperatures, such as indicated above.

It appears that annealing in a reducing atmosphere does not giveentirely desired results. Especially, annealing is performed in aneutral or oxidizing atmosphere. Especially, the method includesannealing in an oxidizing atmosphere. Further, the method may especiallycomprise annealing at a temperature of at least 900° C. for at least 30min. For instance, the temperature may be in the range of 900-1800° C.Note that heat treatment and annealing are two different actions, whichare in general separated by one or more other steps, such as including acooling step. For the synthesis of a layer, the temperature maximum forthe (first) heat treatment and the annealing may be limited to thetemperature stability of the substrate and/or the reactivity of thesubstrate. In general, the temperature should not be higher than 1200°C., such as not higher than 1100° C. For powder synthesis, the annealingtemperature may be above 1000° C., such as at least 1200° C.

The term “substantially” herein, such as in “substantially all emission”or in “substantially consists”, will be understood by the person skilledin the art. The term “substantially” may also include embodiments with“entirely”, “completely”, “all”, etc. Hence, in embodiments theadjective substantially may also be removed. Where applicable, the term“substantially” may also relate to 90% or higher, such as 95% or higher,especially 99% or higher, even more especially 99.5% or higher,including 100%. The term “comprise” includes also embodiments whereinthe term “comprises” means “consists of”.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

The devices herein are amongst others described during operation. Aswill be clear to the person skilled in the art, the invention is notlimited to methods of operation or devices in operation. Hence, thephrase “II-VI based light emitting semiconductor device” is alsodirected to a device which is switched off, and which will in theswitched off state not be light emitting. The semiconductor layercomprising an emissive material may especially comprise an n-typeemissive material. Hence, the semiconductor layer may be an n-typesemiconductor layer, such as n-ZnO or n-CdS, etc.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “to comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

The invention further applies to a device comprising one or more of thecharacterizing features described in the description and/or shown in theattached drawings. The invention further pertains to a method or processcomprising one or more of the characterizing features described in thedescription and/or shown in the attached drawings.

The various aspects discussed in this patent can be combined in order toprovide additional advantages. Furthermore, some of the features canform the basis for one or more divisional applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts emission spectra of a number of luminescent materials;Normalized emission spectra of ZnO (a), ZnO:Al (10 ppm) (b),Zn_(0.9)Mg_(0.1)O (c), and Zn_(0.9)Mg_(0.1)O:Al (10 ppm) (d). Allmeasured as powders sandwiched between quartz plates, excitation at 325nm;

FIG. 2 depicts excitation spectra of a number of luminescent materials;Normalized excitation spectra of ZnO:Al (10 ppm) (b), Zn_(0.9)Mg_(0.1)O(c), and Zn_(0.9)Mg_(0.1)O:Al (10 ppm) (d); and

FIG. 3a-3b depict SEM graphs from a sputtered aluminum doped zincmagnesium oxide layer before (a) and after (b) annealing at 1100° C.

FIGS. 4a and 4b show PL (photoluminescence) spectra of sputteredZn0.85Mg0.15O doped with 40 ppm Al (ZAM-40) deposited on sapphire (4 a)and ITO-coated sapphire (4 b). Films were deposited at room temperature,and were subsequently post annealed; a plurality of annealingtemperatures were investigated; some temperatures are indicated tobetter understand the temperature trends;

FIG. 5 depicts photoluminescence EQE (external quantum efficiency)measurements as function of post deposition anneal temperature forZAM-40 deposited on sapphire;

FIG. 6 shows a comparison of the PL spectra of ZAM-40/sapphire andZAM-40/ITO/sapphire versus post-deposition anneals temperature;

FIGS. 7a (left) and 7 b (right) show PL spectra of ZAM layer on ITOcoated sapphire capped with ZrO layer of different thicknesses. FIG. 7ahas on the left axis the absolute irradiance in photons/cm²·nm); FIG. 7bhas on the y-axis the normalize photoluminescence (in arbitrary units);both have a wavelength scale (in nm) as x-axis;

FIG. 8 shows normalized PL spectra (in arbitrary units on the y-axis) ofsapphire/ITO/ZAM layer (dot-dashed) capped with MgO (line) as functionof the wavelength (in nm);

FIGS. 9a (left), 9 b (right) schematically show an embodiment of thedevice layout of a (ZnO) diode; References I-VI respectively refer tothe anode (I), such as Au and/or Pt, the barrier layer (II), such as ametal oxide, having a layer width of larger than 0 nm and e.g. equal orsmaller than 30 nm, the active layer (III), such as (Zn.Mg)O:Al, thecathode (IV), such as ITO, electrode(s) (V), such as Pt electrodes, anda substrate (VI), such as sapphire, quartz or glass;

FIGS. 10a (top), 10 b (bottom) show: FIG. 10a (top) I-V characteristicsof ITO/ZAM/MgO/Au diode; FIG. 10b (bottom) electroluminescent spectra ofthe diodes driven at 10V and 50 mA. The peak shown with the arrow showsthe presence of near-band edge emission (NBE) of ZnO indicating holeinjection into ZAM layer; in FIG. 10b , the curve with two peaks is the10V/50 mA electroluminescent spectrum; the other curve is the PLspectrum (see e.g. FIG. 8);

FIG. 11 shows EL spectra of ITO/ZAM/MoOx/Au; ZnO NBE again indicates thenear-band edge emission (NBE) of ZnO; the curve that is higher at 500 nmbut lower at 900 nm is the PL (thin film) spectrum; the other curve(with more fluctuation on the signal) is the 12.5 V EL spectrum;

FIGS. 12a (top), 12 b (bottom) show I-V characteristics (top) and ELspectra taken at 10 V (bottom) of ZAM devices with a ZrO blocking layer;in FIG. 12b , PL refers to photoluminescence, EL BA refers toelectroluminescence before annealing and EL AA refers toelectroluminescence after annealing;

FIG. 13 schematically shows an embodiment of energy band diagrams of then-ZnO/SiOx/p-type Si diodes under thermal equilibrium (left) andpositive bias at Si side (right).

DETAILED DESCRIPTION OF THE EMBODIMENTS

For the compositions Zn_((1-x))Mg_(x)O desired quantities of zinc oxide(5N purity, Aldrich) and magnesium oxide (FO Optipur, Merck) wereweighed into a 100 ml beaker and mixed for 4 minutes at 1800 rpm using aspeed mixer (Hauschild, type DAC 150 FVZ-K). The compositions were putinto an aluminum oxide crucible and fired inside a chamber furnace inair for 8 hours at 1100° C. using a heating and cooling rate of 200°C./hour. After cooling down the powders were grinded using an agatemortar and pestle and fired once again at 1100° C.

For aluminum doped Zn_((1-x))Mg_(x)O first a desired amount of aluminumnitrate nonahydrate (p.a., Merck) was dissolved in a small amount ofdeionized water and diluted with 200 ml ethanol. Next desired amounts ofzinc oxide (5N purity, Aldrich) and magnesium oxide (FO Optipur, Merck)were added and the obtained suspension was dried using a rotaryevaporator. The compositions were put into an aluminum oxide crucibleand fired inside a chamber furnace in air for 8 hours at 1100° C. usinga heating and cooling rate of 200° C./hour. After cooling down thepowders were grinded using an agate mortar and pestle and fired onceagain at 1100° C. From the Zn_(0.9)Mg_(0.1)O+10 ppm Al powder, targetssuitable for sputtering and pulsed laser deposition (PLD) were prepared.

A number of 400 nm thin films were grown on epi-polished a-cut sapphiresubstrates by PLD and RF magnetron sputtering. The base pressure of thePLD system was 2×10⁻⁷ mbar. During the deposition the substratetemperature was between 25° C. and 550° C. and the partial oxygenpressure was 0.2 mbar. The RF magnetron sputtering system had a basepressure of 6×10⁻⁷ mbar and the used substrate conditions were either25° C., 450° C. or 550° C. The gas flows during the sputtering processwere resp. 78 and 2 sccm for Ar and O₂, the total pressure was 0.038mbar, RF power was 60 W.

The thin film composition was analyzed using x-ray fluorescence (XRF)and secondary ion mass spectrometry (SIMS). For optical analysis of thepowders, they were sandwiched between Asahi quartz substrates (that werefound to be non-luminescent with at the excitation wavelengths used) andthe sides were sealed with a UV-transparent epoxy glue (Epo-Tek 305).UV/Vis spectra were measured on a Perkin Elmer Lambda-950 spectrometer,emission and excitation spectra on an Edinburgh FLS920 fluorescencespectrometer. Photoluminescence (PL) emission spectra were measured on ahome-built setup consisting of a Ocean Optics QE65000 spectrometeroperating at −20° C., with either a 25 mW 325 nm CW He—Cd laser or aSpectraphysics Explorer 349 nm Nd:YLF pulsed laser as excitationsources. The latter laser was operated at 2.5 kHz repetition rate with apulse length of ˜5 ns. The power incident on the sample was tuned with aVBA-200 beam splitter from Jodon Laser combined with a set of neutraldensity filters. Emission was detected at 90° angle to the incidentlaser beam by collection with a collimating lens, passed through along-pass filter to remove residual laser light and then focused into anoptical fiber connected to the spectrometer. The sample was oriented ata 120° angle with respect to the incident beam to prevent the specularreflection of the laser beam from entering the collimating lens.Absolute external quantum efficiencies were determined using a 6″integrating sphere from Labsphere (model RTC-060-SF) which was equippedwith a center mount. The laser only spectrum was taken with the centermount rotated parallel to the beam, so that the beam did not touch thesample mount directly. For the sample measurement, the beam hit thesample at 10° C. rotated with respect to the normal of the samplesurface, so that the specular reflection of the laser beam was keptinside the sphere. Spectrometer, optical fibers and integrating spherewere all calibrated with a LS-1-CAL calibration lamp from Ocean Optics,to enable absolute irradiance measurements. Cathodoluminescence (CL) wasmeasured on a modified SEM. All optical characterizations were conductedat room temperature.

Normalized PL emission spectra of ZnO+10% Mg (curve c), ZnO:Al (10 ppm)(curve b) and Zn_(0.9)Mg_(0.1)+10 ppm Al (curve d) are shown in FIG. 1.The PL from pure ZnO (curve a) is the typical near-band gap emission(NBE) at ˜385 nm, with a very shallow, broad emission in the visiblethat is generally attributed to (oxygen related) defects. For all theother samples the situation is reversed and the primary PL signal is abroad emission in the visible, centered around 500-600 nm, againattributed to defect emission. This visible emission is specifically notoriginating from direct luminescence of the dopants themselves. Someminor NBE signal is visible in the UV. It is especially noteworthy thatthe addition of only a small amount of Al (10 ppm) (curve b) changes thePL output completely from almost entirely NBE-emission to almostentirely defect emission, with the peak maxima remaining virtuallyunchanged from the host ZnO.

Some differences in the wavelength of maximum visible emission betweenthe different powders are observed: 520 and 585 nm for Zn_(0.9)Mg_(0.1)O(curve c) and ZnO:Al (curve b), respectively, with the sample accordingto the invention being in the middle at 555 nm (curve d).

The excitation spectra of ZnO+10% MgO (curve c), ZnO:Al (10 ppm) (curveb) and Zn_(0.9)Mg_(0.1)+10 ppm Al (curve d) as measured with anEdinburgh fluorescence spectrometer are shown in FIG. 2. The excitationspectrum of ZnO could not be measured due to the very low emission. Itcan be seen that the optimal excitation wavelength is about 385 nm forthe non-Mg containing powder which coincides with the NBE emission ofZnO. The Mg containing samples have their optimal excitation wavelengthat 350 nm, but in both cases a secondary peak is observed at 385 nm,which is especially high in the ZnO/Mg case. This secondary maximum isagain indicative that the ZnO and MgO have not fully mixed.

Table I shows the results from absolute (external) quantum efficiency(EQE) measurements on ZnO powders with various amounts of Mg and/or Al,measured at 349 nm excitation. Absorption at this wavelength istypically about 85% of the incident light. The power of the laser wastuned so as to be in a regime where the emission varied linearly withthe intensity. It is immediately clear that having none or only one ofMg and Al present in the powder results in only limited quantumefficiency. When both are present, a large increase in EQE is observed.

TABLE I External quantum efficiencies (%) of Zn_((1-x))Mg_(x)O: Alpowders as a function of composition. Excitation with 349 nm laser. % Mgppm Al 0 1 5 7.5 10 15 20 0 0.8 2.1 3.0 2.5 10 2.5 5.6 13.7^(a) 8.7 2014.7 40 15.3 23.7 70 10.6 100 2.0 9.4 1000 1.1 8.0 ^(a)an earlier batchof powder, that was used to prepare the target for PLD and sputtering,was found to have an EQE of 9.8%.

The dependence on the Al content is intriguing. Only a tiny amount (˜10ppm) is needed to increase the EQE of the ZnO/Mg powder, and adding(much) more has no substantial effect or may lead to other undesiredproperties, like a too large electric conductivity. Hence, an amount ofat maximum 200 ppm, especially at maximum 100 ppm seems beneficial.

Normally for a phosphor at low activator content, the PL outputincreases linearly with doping content as the emission competes withnon-radiative processes in the host lattice. This linearity generallyremains until concentration quenching sets in, typically above a fewpercent dopant, as at such higher concentrations the dopant centersstart to interact by processes like Auger recombination. The dependenceon Mg content is also found to be non-linear.

From the Zn_(0.9)Mg_(0.1)O+10 ppm Al 400 nm thin layers were depositedon sapphire substrates by PLD and RF sputtering. Analysis of thesputtered layers by XRF and SIMS showed the Mg and Al content to be 9.6%and 14 ppm respectively, so the concentration of both dopants is more orless preserved during the deposition process. X-ray analysis showed bothdeposition techniques to afford essentially epitaxial layers.

While the layers were deposited at elevated substrate temperatures (500°C. for PLD, 450° C. for sputtering), the PL of the as deposited layerswas low. It was found that annealing of the samples was required toachieve maximum luminescence, as is shown for both types of deposition.The minimum temperature for maximum PL appears to be 900° C. for bothsamples, although there is a marked difference in the evolution of thePL as a function of anneal temperature for the two depositiontechniques.

For the PLD sample, at 700° C. there appears to be an intermediate stagewhere 2 peaks are visible in the PL spectrum. After anneal at 900° C.,the spectrum is more or less identical to that of the parent powder.Above 900° a slight apparent increase in PL output could still beobserved. The sample itself however exhibited formation of a haze in theformerly transparent sample according to the invention layer. SEM showedthis haze to be due to the presence of slightly larger ‘crystallites’that have grown at elevated temperatures. Cracks were not observed. Thishaze affect is likely to lead to a more efficient outcoupling of thelight normal to the plane of the sample according to the invention layer(where the PL emission is measured). The sputtered layers were found toremain clear upon annealing up to 1100° C. SEM pictures from a sputteredaluminum doped zinc magnesium oxide layer before (a) and after (b)annealing at 1100° C. are shown in FIG. 3 (a and b, respectively).

In order to answer the question if annealing at higher than 900° reallyresults in higher output or if the hazing effect clouds the issue, forboth types of deposition techniques the absolute EQE as a function ofanneal temperature was also determined. The results are listed in TableII, and indeed the EQE at 1000° C. anneal is slightly lower than at 900°C. (although the values are close to the detection limit). A similaranneal experiment was performed for Zn_(0.85)Mg_(0.15)O+40 ppm Al wherea similar trend was observed, as well as higher EQE values. The optimumtemperature was found to be 950° C., in line with the data forZn_(0.9)Mg_(0.1)O+10 ppm Al.

Table II reflects systems wherein the layers have the nominalcomposition Zn_(0.9)Mg_(0.1)O:Al (10 ppm) and Zn_(0.85)Mg_(0.15)O:Al (40ppm).

TABLE II Absolute EQE (at 349 nm excitation) for samples according tothe invention layer deposited on sapphire, versus anneal temperature.Anneal performed in air for 30 minutes. Anneal Absolute QE (%) AbsoluteQE (%) Temperature Zn_(0.9)Mg_(0.1)O: Al Zn_(0.85)Mg_(0.15)O: Al (° C.)(10 ppm) (PLD) (40 ppm Al) (sputter)  500 (as deposited) 0.26  700 0.55 900 1.10 1.64  950 7.23 1000 0.97 6.13 1050 4.32 1100 0.9 1150 0.46

In the case of the sputtered layer, two things become apparent. Firstly,the wavelength of maximum emission is red shifted some 50 nm withrespect to the parent powder emission. Secondly, upon annealing atincreased temperatures, a second peak starts to appear at 480 nm. Uponfurther annealing, the 480 nm peak starts to disappear again and aslight blue shift of the main peak is observed. At the highest annealtemperature (1100° C.) the 480 nm peak is completely gone and the mainpeak has shifted to 550 nm. The resulting PL spectrum is completelyidentical to a powder sample according to the invention. It appears thatsputtering results in different phases in the layer, and annealing at1100° C. is gives best results.

Apart from the temperature, the effect of the annealing atmosphere wasalso checked. Identical samples of sample according to the invention onsapphire, deposited by deposition, were annealed in differentatmospheres (neutral, reducing and oxidizing) for 1 hour at 650° C. Notethat this lower temperature was dictated by the requirements of one ofthe electrode materials (ZnO+2% Al). The PL output was measured usingthe qualitative part of the setup as the EQE's were generally below thedetection limit of the quantitative setup. As the outcouplingcharacteristics of the samples were similar, this still affords a goodcomparison of the emission. For most atmospheres, the maximum emissionwas observed at 610 nm. In several samples a shallow shoulder wasobserved at 790 nm that was especially visible in the vacuum annealedsample. The relative results of the PL output are listed in Table III,with the sample annealed for 1 hour in air set at 100%. Theconductivities of the layers were also determined.

TABLE III relative PL output and conductivity of PLD samples accordingto the invention-10 layers on sapphire, as a function of the annealatmosphere. Anneal done for 1 hour (unless stated otherwise) at 650° C.and atmospheric pressure. Relative photon Sheet resistance Annealatmosphere flux (%) (MQ/square) As deposited (500° C.) 0.6 1E+5 Air (1hour) 100.0 <1E+4   Air (64 hour) 96.8 3E+4 Argon 97.5 4E+1 Oxygen 64.13E+4 5% hydrogen in argon 1.1 8E+4 vacuum 42.3 1E+1 NH₃ 6.7 1E+4Nitrogen (dry) 83.8 1E+1

From Table III it is clear that ambient air affords the best performingsamples for PL output. Upon annealing for prolonged periods of time inair, a slight decrease in performance is observed as well as a smallredshift of the emission to about 630 nm. The ‘neutral’ atmospheresargon and nitrogen provided results similar to air. Vacuum and pureoxygen, had roughly half the output of the air sample, presumably byboth influencing the (number of) oxygen vacancies in a non-ideal way.The reducing atmospheres (H₂/Ar and NH₃) had severely diminished output,presumably by removal of oxygen from the sample according to theinvention layer.

The sheet resistance of the layers was generally high (10-100 GΩ/square)for all atmospheres barring the ‘neutral’, non-oxygen containing ones(vacuum, argon, nitrogen) where it was 3 orders lower.

Hence, a new type of zinc oxide based phosphors has been prepared byincorporating both MgO (e.g. up to 15%) and a trace (e.g. 10-40 ppm) ofAl as dopants. These phosphor powders showed visible emission and anorder of magnitude increase in quantum efficiency compared to ZnO withno or only one of Mg and Al present. The phosphors proved robust to thinlayer deposition techniques such as PLD and RF sputtering. Annealing inair at elevated temperatures (up to 900-1100° C. depending on thedeposition technique) was found to be very beneficial for integration ofall the substituent materials in the thin layers and increase thephotoluminescence. The enhanced emission in both powder and thin layercould not be attributed to direct emission of the additives, but isthought to stem from radiating defects in the ZnO lattice, most likelyoxygen-related. Only band edge excitation was observed, which wasfurther corroborated by CL, showing that these phosphors operate throughenergy absorption by the host material, followed by energy transfer tothe radiant defect and subsequent emission, making these materialspotential candidates for the emissive layer in large area LEDs.

Herein, we further present a generic solution toward achievinglight-emission from devices that are made of thin-films of ZnMgO:Alphosphor sandwiched between two/or more layers. Functional ZnO LEDs aredemonstrated, with EL spectra that match that of the ZnO phosphor.

For detailed preparation of emissive material we refer to the above.Here a short explanation of the phosphor preparation is given. Foraluminium doped Zn_((1-x)) Mg_(x)O first a desired amount of aluminiumnitrate nonahydrate (p.a., Merck) was dissolved in a small amount ofdeionised water and diluted with 200 ml ethanol. Next desired amounts ofzinc oxide (5N purity, Aldrich) and magnesium oxide (FO Optipur, Merck)were added and the obtained suspension was dried using a rotaryevaporator. The compositions were put into an aluminium oxide crucibleand fired inside a chamber furnace in air for 8 hours at 1100° C. usinga heating and cooling rate of 200° C./hour. After cooling down thepowders were grinded. After firing once again at 1100° C., targetssuitable for sputtering and pulsed laser deposition (PLD) were prepared.

Thin films of ZnO phosphor were RF magnetron sputtered on a variety ofsubstrates. Thin films of other metal oxides were either sputtered ofphysical vapor deposition. First 400 nm thin films of ZnO phosphor wasgrown on ITO coated epi-polished a-cut or c-cut sapphire substrates byPLD or RF magnetron sputtering. The base pressure of the PLD system was2×10⁻⁷ mbar. During the deposition the substrate temperature was between25° C. and 550° C. and the partial oxygen pressure was 0.2 mbar. The RFmagnetron sputtering system had a base pressure of 6×10⁻⁷ mbar and theused substrate conditions were either 25° C., 450° C. or 550° C. The gasflows during the sputtering process were resp. 78 and 2 sccm for Ar andO₂, respectively. The total pressure was 0.038 mbar, and the RF powerwas 60 W, and the bias voltage was around 250V. Next a layer ofmetal-oxide was deposited on to the phosphor layer and then metalcontacts were deposited. Devices were annealed and then measured.

Photoluminescence (PL) emission spectra were measured as defined above.

Electrical measurements were conducted in a dark chamber at ambient.Light emission from the devices was recorded using a photo-diode.Current-voltage characteristics of the diodes were recorded using HPsemiconductor analyzer. To record the EL spectrum of the LED, the OceanOptics QE65000 spectrometer operating at −20° C. was used. The emittedlight from the LED was fed into an optical fiber that was mounted on topof the emissive area and connected to the spectrometer.

Sputtered Thin Layers

The RF magnetron sputtering was used to sputter thin films of differentvariation of Zn_(0.90)MgO_(0.10) (ZAM-10) and Zn_(0.85)Mg_(0.15)O. Thephosphors used here were doped with Al in range of 0 to 100 ppm. Therange of Al doping can be higher. The substrate temperature could becontrolled during the deposition. Many phosphor compositions were made,measured and used in devices. Thin-film deposition conditions werevaried, e.g. substrate temperature, from RT, to ˜500° C. Here we onlypresent the results on the Zn_(0.85)Mg_(0.15)O doped with 40 ppm Al(ZAM-40) deposited at RT.

Thin film sputtering was conducted at a base pressure of 6×10⁻⁷ mbar.The substrate temperature during deposition was kept at roomtemperature. The RT substrate temperature was justified by ourinvestigation that showed samples having different substrate depositiontemperature have similar PL after annealing at T>550° C. Hence thechoice of low substrate temperature is justified.

Sputtered films were prepared on Sapphire and ITO-coated Sapphiresubstrates. After deposition each substrate was subjected to annealingat one particular temperature. Thus no thermal histories were presentfor the samples. The annealing temperature was varied between RT up to1150° C. for 30 min in ambient. After annealing samples were cooled downrelatively slowly for 10-15 min in ambient air. Subsequently PL and EQEwere measured. Later XRD and AFM were performed.

Primary results of the PL measurements are given in FIG. 4a-4b , wherePL is measured as a function of post-annealing for RT sputtered thinfilm of Zn_(0.85)Mg_(0.15)O doped with 40 ppm Al, on the sapphire andITO-coated sapphire substrates. Deposition of the phosphor layer at roomtemperature results in low PL emission as shown in the insets of FIG.4a-4b . It is clear that post-annealing of the films have a profoundinfluence on the PL spectra, as the emission enhances with increasingannealing temperature. However there is an optimum for the annealtemperature. It seems that there is an optimum annealing temperature isbetween 900-1000° C., where the PL response maximizes.

The optimum of post-anneal temperature for ZAM/sapphire was determinedby EQE measurement of the different samples. The results of the EQEmeasurements as function of temperature, is given in FIG. 5. It seemsthat the best annealing temperature for ZAM-40/sapphire is 950° C.,where EQE exceeds 7.2%. EQE of the sputtered thin-film of ZAM-40 isalmost a factor of 2 larger than that of the epitaxially grown GaN (4%).

In fabrication of the LED however the ZAM layer is deposited on toanother layer of either metal or metal-oxide which acts as the electrodefor charge injection into the device. Therefore PL response of the ZAMlayer could be different. To this point ZAM-40 was deposited ontoITO-coated sapphire. PL spectra is given in FIG. 4a . The only effect ofthe ITO seems to be red-shifting the defect emission peak of the ZAM-40from 550 nm to >600 nm. The initial red-shift gradually decreases towardthe original defect emission of ZAM-40 (FIG. 4a ) as the annealingtemperature rises. At 900° C. however the shift of the defect emissionpeak toward lower wavelengths stops and PL abruptly changes. This abruptchange in the PL spectra has to do with the fact that ZAM-40 attemperatures higher than 900° C. start to form alloy with ITO hencechanges the PL spectra. It is however of interest to see whetherpresence of ITO hampers the light emission from the ZAM-40 layer. To doso, we calculated photon flux emitted from ZAM-40 deposited on bothsapphire and ITO-coated sapphire and compared both.

In FIG. 6 a comparison has been made for the photon flux (PF) of the ZAMon sapphire, and ITO/sapphire. Presence of the ITO does not compromiseon the optical performance of the ZAM layer up to 800° C. At the sametime this figure shows that annealing temperatures in the range of 400°C. to 800° C. have a very negligible influence on the PL emission of theZAM. At 400° C. the phosphor is already activated. The lower performanceof the ZAM/ITO/sapphire in compare to ZAM/sapphire, at temperatureshigher than 800° C. is due to the degradation of the ITO and possiblyformation of ZAM:ITO alloy. For ZAM/sapphire substrates, there is a risein photon flux with a maximum at around 950° C.-1000° C., indicating theoptimum annealing temperature. Surprisingly, light emission from bothsamples is the same and the best of phosphor activation is reached whenZAM is annealed up to 950° C.-1000° C. ITO however cannot withstandthese high temperatures. Application of metals or conducting metal-oxidewhich can stand high annealing temperature would be advantageous in thisrespect, as it allows full activation of phosphor in real devices.

PL Spectra of ITO/ZAM/Insulating-Oxide Stack

The first question to be addressed here is whether deposition of anextra oxide layer would change the emission spectra of the ZAM layer. Todo so, we sputtered ZAM onto the ITO-coated substrate. As a test model,we deposited 5 nm and 10 nm of ZrO onto the ZAM layer. The substrateswere annealed at 600° C. for 30 min and slowly cooled down. Therespective PL spectra of the samples are shown in FIG. 7a-7b . Clearlyinsertion of the ZrO layer does not change the PL spectra. The intensityhowever seemingly drops slightly in the presence of ZrO layer. Excludingall the experimental and instrumental errors, one possible reason willbe less light out-coupling when an extra ZrO oxide layer is incorporatedonto the stack.

To further investigate whether the top insulating layer influences thePL of the ZAM layer, we deposited MgO layer onto the ZAM layer andsubsequently annealed the stack at 800° C. FIG. 8 shows the PL spectraof the ZAM layer capped with MgO layer in comparison with a bare ZAM.Clearly there is no influence of the insulating MgO layer on the PL ofthe phosphor even after annealing at 800. Incorporation of an insulatinglayer into the diode stack therefore has no influence on the PL spectraof the emissive ZAM layer. In fabrication of the diodes we thereforetried different insulating metal oxides such as, MgO, MoOx, V₂O₅,NiO_(x) and ZrO. Experiments with SiO₂ (SiO_(x)) and other oxides werealso conducted, and similar results were obtained.

Fabrication of ZnO LEDs

Here, a diode is realized by incorporation of an insulating layer in thedevice stack, i.e. metal-insulator-semiconductor-metal (MISM) diode.Typical diode layout is shown in FIGS. 9a-9b . However, otherconfigurations may also be possible (including an inverted structure).

In the following we present the data obtained for MISM ZnO diodefabricated with the sputtered thin films of Zn_(0.75)Mg_(0.15)O dopedwith 40 ppm Al. We used different substrates, e.g. sapphire, quartz andglass. Here only the results of devices fabricated on sapphire substrateare presented. The operation mechanism of the diode is discussed in thelater section.

As cathode we used both Al doped ZnO and ITO both sputtered onto thesubstrate. We note that any metal, or transparent conductive metal-oxidecan be used as cathode. ZnO:Al however is advantageous as it provides agood template for ZAM growth. In most of our experiments we used ITO ascathode. Thermal annealing at temperatures˜600° C. was performed toactivate the phosphor. Sputtered ITO on glass did show very littledegradation in sheet conductivity upon annealing up to 750° C.Conductivity varied from 30 at RT to 75 Ω/square for ITO annealed at750° C. Glass however, is not stable at T>700° C. Therefore we usedeither ITO coated sapphire or ITO coated quartz as substrate for ZAMgrowth and device fabrication.

In the next step we introduced the Pt pad on the ITO-coated sapphirewith shadow mask evaporation followed by ZAM deposition. We note that itin our experiments the Pt-cathode pads were masked from the ZAM layer.We do not expect however significant differences if the Pt-contact padsare in touch with the ZAM layer. In the next step either a combinationof metal contacts e.g. Ni/Au, or a combination of metal-oxide/metalcontacts were introduced as anode. Later annealing of the device wasperformed to activate the phosphor and to form the contact.

We note that annealing of the devices is another crucial step in devicefabrication. In order to fabricate reproducible device, first thecontacts were deposited and then annealed at the desired temperature.Subsequent slow cooling down process of the substrate to RT is vital.Rapid cooling of the sample or deposition of contact after annealing,both resulted in devices with symmetric I-V characteristics and no lightemission.

Here we present the results obtained with magnesium oxide (MgO),molybdenum oxide (MoOx), vanadium oxide (V2O5) and zirconium oxide(ZrO). We note that the same results were obtained with other insulatingblocking layers in combination with different anodes. Moreover ZnO LEDswith the MISM layout can also be fabricated in an inverted structure. Anexample would be ZAM deposited onto p-type Si with a few nm thickSiO_(x) oxide layer.

Electrical Characterization of ZnO LEDs

In this section we present electrical characteristics of MISM ZnOdiodes. Current-voltage characteristics and electroluminescent spectrafor sapphire/ITO/ZAM/MgO/Au diode are given in FIGS. 10a-10b . The I-Vcharacteristic of the device, measured in dark, shows that the diode isrectifying. The rectification ratio however is not large due to theleakage current. The primary target here is demonstration of afunctional diode and electroluminescence. A photo-detector (photo-diode)was placed over the ZnO diode to record the light emission of thedevice. In dark we measured light with the photo diode. The powerdissipated in the ZnO LED was less than 0.5 W, hence just not enough torecord a measurable black body radiation. We measured theelectroluminescent of the ZnO LED in forward bias of 10 V. The currentrunning through device was 50 mA. An EL spectrum of the device is givenin FIG. 8. The PL spectrum of the ZAM is also presented as a reference.

FIGS. 10a-10b show nice matching of the EL spectrum of the ZnO LED withthe PL of the ZAM thin film. It is really intriguing to note that the ELshows a peak at 358-360 nm. This EL peak is exactly at the positionwhere near-band edge emission of the ZnO in thin films of ZAM takesplace. Moreover the peak at 670 nm also nicely matches with the emissionof the ZAM phosphor. Presence of 358-360 peak unambiguously demonstratethat hole injection is achieved with the MISM structure. The MISM devicelayout is therefore viable to overcome the challenge that has impedingarriving at ZnO LED for more than 60 years. To further prove that theMISM concept is generic for ZnO LEDs, in the next step we used MoOx as ablocking layer. The device layout therefore wassapphire/ITO/ZAM/MoOx/Au.

In FIG. 11. we have only presented the EL spectra of the device. El wasmeasured at 12.5 V and a good spectral match between the PL of ZnO thinfilm and EL is achieved. Once again, presence of the near-band edgeemission in the EL spectra indicates successful achievement of holeinjection into the valance band of the ZnO.

It is highly desirable to have a blocking layer that first, is stable inair, and second, does not intermixes with the underlying ZnO phosphorlayer upon annealing. A good candidate for such layer is ZrO, which is astoichiometric oxide with very limited solubility in ZnO. ZnO diodeswere fabricated with ZrO blocking layer. The device stack wassapphire/ITO/ZAM/ZrO/Au. ZrO layer was sputtered from Zr target in anoxidizing atmosphere. FIGS. 12a-12b . show the I-V characteristics andEL spectra of the respective ZnO diodes. The diodes show an excellentrectification behavior as well as decent light emission. The powerdissipated in the diode was ˜50 mW. The emitted light therefore cannotbe infra-red emission due to heat dissipation, as shown by the ELspectra of the device. EL spectra were recorded before and afterannealing of the diode. Before annealing, EL shows a peak at 900 nm, andit is not due to heat dissipation. Upon annealing, several peaks appearin the EL spectra of the device, with the first peak being at ˜650 nm.In comparison with the PL spectra of the thin-film with ZrO layer ontop, it seems that the main emission peak at 600 nm is red shifted.Moreover the near band edge emission peak of the ZAM layer is notpresent in the EL spectra. Additionally few other peaks are present inthe EL. We believe that sputtering of the ZrO layer on to ZAM has causeddamages at the ZAM/ZrO interface. Due to soft bombardments of the ZAMinterface, shallow diffusion of the Zr or ZrO into the ZAM layer couldpotentially change the EL spectrum by causing more surfacerecombination, which is manifested by appearance of the new peaks.Physical vapour deposition of ZrO (and potentially all the blockinglayers) onto the ZAM layer is therefore recommended for having goodspectral match.

The EL-spectra presented here are among the first EL spectra reportedfor ZnO LEDs. The I-V characteristics and EL spectra achieved for theZnO LEDs demonstrate the viability of the MISM device layout.

Light-Emission Mechanism of ZnO LEDs

A tentative mechanism is presented in FIG. 13 for the MISM stack withhighly p-type doped Si as the anode and SiOx as the blocking contact.Similar mechanism is at work when p-type Si is replaced with a metalelectrode as anode.

The energy band diagrams of the diode at equilibrium and under bias areshown. When positive forward bias is applied on the anode, here p-typeSi, the bands of Si near the Si/SiO_(x) interface will bend upward. Theband bending at the Si/SiO_(x) interface will gradually induce aninversion layer for n-ZnO/SiO_(x)/p-Si diodes, which is responsible forthe hole injection. As a result, accumulated holes in the inversionlayer could tunnel through the barrier into the valence band of ZnO andrecombine with the electrons in ZnO conduction band that are blocked bythe SiOx interface layer, resulting in UV emission of 359 nm as well asthe visible emission at 600 nm.

A zinc oxide light emitting diode based on a newly developed zinc oxidephosphors has been demonstrated. These phosphor thin film showed visibleemission. The phosphors proved robust to thin layer depositiontechniques such as PLD and RF sputtering. Annealing in air at elevatedtemperatures (400-1100° C.) was favorable to increase thephotoluminescence and initiate the electroluminescence. To fabricate ZnOLED we used a blocking layer between the anode and the emissive layer.The blocking layer impedes the electron to arrive at the anode from theZnO layer. Accumulation of the electron enhances hole injection andhence the LED begin the light emission.

The recorded electroluminescence and the photoluminescence spectra ofthe ZnO thin film and ZnO LED match nicely. Interestingly even band gapemission of the ZnO is present in the EL spectra, which indicates thathole injection has been successfully achieved by incorporation of theblocking layer. The enhanced emission in ZnO thin layer could not beattributed to direct emission of the additives, but is thought to stemfrom radiating defects in the ZnO lattice, most likely oxygen-related.Only band edge excitation was observed, which was further corroboratedby CL, showing that these phosphors operate through energy absorption bythe host material, followed by energy transfer to the radiant defect andsubsequent emission. The combination of the material and devicepresented here makes ZnO phosphors an attractive potential candidate forthe large area LEDs.

As insulating layers, SiO₂, MgO and ZrO were tried, and they all worked.

The invention claimed is:
 1. A light emitting semiconductor devicecomprising: a stack, the stack comprising: a cathode, a semiconductorlayer comprising an emissive material having an emission in the range of300-900 nm, an insulating layer, and an anode, wherein the cathode is inelectrical contact with the semiconductor layer, wherein the anode is inelectrical contact with the insulating layer, wherein the insulatinglayer has a thickness in the range of up to 50 nm, wherein thesemiconductor layer comprises aluminum doped zinc magnesium oxide layerhaving 1-350 ppm Al.
 2. The light emitting semiconductor deviceaccording to claim 1, wherein the emissive material has a conductionband at CBp eV and a valence band at VBp eV from the vacuum level, withCBp>VBp, wherein the insulating layer has a conduction band at CBb eVand a valence band at VBb eV from the vacuum level, with CBb>VBb,wherein CBb>CBp and wherein VBb≦VBp+1.5 eV.
 3. The light emittingsemiconductor device according to claim 1, wherein the semiconductorlayer comprises an emissive material selected from the group consistingof ZnO, (Zn,Mg)O, ZnS, ZnSe, CdO, CdS, CdSe, and doped variants of anyof these.
 4. The light emitting semiconductor device according to claim1, wherein the semiconductor layer has a nominal compositionZn_(1-x)Mg_(x)O with 1-350 ppm Al, wherein x is in the range of 0<x≦0.3.5. The light emitting semiconductor device according to claim 1, whereinthe insulating layer is an oxidic insulating layer selected from thegroup consisting of SiO₂, MgO, SrTiO₃, ZrO₂, HfO₂, and Y₂O₃.
 6. Thelight emitting semiconductor device according to claim 1, wherein theinsulating layer has a thickness in the range of 2-30 nm.
 7. The lightemitting semiconductor device according to claim 1, wherein thesemiconductor layer has a nominal composition Zn_(1-x)Mg_(x)O with 1-350ppm Al, wherein x is in the range of 0.02<x≦0.2.